The present invention relates to methods and apparatus for fracturing rock, ceramics, concrete and other materials of low elasticity. The invention relates in particular to methods and apparatus for fracturing rock for purposes of mining, excavation, and demolition.
Mining and excavation of rock is commonly carried out using explosives. Typically, sticks of explosive are placed in holes drilled into the rock and then detonated, thereby explosively fragmenting a portion of the rockface being worked on. The rock debris created by the explosion is cleared away, and preparations begin for another blast.
The blasting method described above is time-consuming and expensive. Each blast takes a considerable time to set up and carry out. A large number of holes must be drilled into the rockface and the explosives placed in the holes, carefully interconnected with fusing apparatus to ensure that they detonate simultaneously. The resultant blast can throw rock debris large distances, unless the configuration of the blast is such that heavy and expensive blasting mats can be put in place to cushion the explosion and prevent the blast debris from flying away. As with any operation employing explosives, the blasting method also is inherently hazardous to the persons involved.
Accordingly, there is a need for rock mining and excavation methods, which are faster and more efficient and thus less expensive than conventional blasting methods. There is also a need for rock mining and excavation methods, which eliminate or substantially reduce the safety hazards associated with conventional rock blasting practices.
One possible alternative to conventional mining methods is to fracture the rock by means of thermal stress. It is well known that solid materials can fracture due to internal stresses induced by a large and sudden temperature change. A simple example of this is the shattering of a piece of glassware plunged into cold water after having been heated. Similarly, rock will shatter if it undergoes a temperature rise great enough and sudden enough to induce internal tensile or shear stresses exceeding the inherent tensile or shear strength of the rock. This would be a desirable result for purposes of rock mining and excavation. Material near the surface of a rock mass would be heated rapidly, and resultant thermal stresses would fracture the rock. The fractured material may then be removed, and the process repeated on the fresh rock thus exposed, and so on until a desired amount of rock has been removed.
The practical difficulty with this concept, of course, is how to create such a sufficiently sharp and intense temperature rise in the surficial zone of a rock mass, before the heat thus transferred to the rock can be dissipated by conduction throughout the rest of the rock mass. Conventional flame-heat sources, however, are not capable of achieving the desired result. An acetylene-oxygen flame, for example, can achieve a maximum temperature of approximately 3,100° C., but tests have indicated that even a flame this hot is not effective for producing thermal stresses intense enough to fracture rock effectively.
In U.S. Pat. No. 3,826,537 to Boyd, a tunnelling apparatus includes both thermal and mechanical energy. The rock is heated with tungsten filament infrared lamps and then subjected to an impactor in order to excavate the rock. Tungsten filament lamps may produce temperatures of about 2200° C. (4000° F.). Again, these sources of heat are insufficient to reliably fracture rock unless it is susceptible to fracture by containing large amount of impurities or water. At these slower rates of heating, tensile stresses only are produced in the rock, resulting in deep fissures or cracking. These tensile cracks may not permit efficient excavation in a tunnelling procedure and in fact may damage the tunnel wall strength. Efficient excavation may only take place with a combination of thermal and mechanical energy.
Accordingly, there is a need for improved methods of fracturing rock or other brittle materials using a radiant energy source.